In one of the rare times I had my iPhone 3GS out of its leather case, I dropped it on a tile floor. The chrome Power Button came loose and disappeared, forcing me to have to sleep the iPhone by trying to depress the remaining part of the button with a fingernail - not easy.
I looked at the instructions for replacing the switch on iFixit.com, which were certainly complete, but would have required quite a bit of delicate work.
I spotted in the remaining part of the switch a slot in which the chrome button cap must have been pressed. Since the cost of the part, through iFixit.com, was quite affordable ($5), I ordered one. The part arrived quickly. Upon its arrival, I easily - by hand - was able to pull off the chrome button cap from the new switch assmebly and, simply, press it into place where the original had been.
Easy repair, without requiring disassembly of the phone.